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Extending LANs

Extending LANs Fiber modems Repeaters Bridges Switches Distance limitation in LANs MAC protocols such as CSMA/CD require time proportional to the length of the cable Electrical signal weakens with distance

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Extending LANs

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  1. Extending LANs • Fiber modems • Repeaters • Bridges • Switches

  2. Distance limitation in LANs • MAC protocols such as CSMA/CD require time proportional to the length of the cable • Electrical signal weakens with distance • Network designers choose a combination of capacity, delay and distance that can be achieved within a given cost

  3. Fiber optic extensions • Fiber modems extend connection between computer and transceiver

  4. Repeaters • Join Ethernet cables (segments) together • Amplified signal - no knowledge of frames • Deals with signal strength, but not delay

  5. Multiple repeaters • Ethernet standard says no more than four repeaters between two computers

  6. Fiber modems can be used between repeaters for long distance extensions • Biggest problem with repeaters is that they transmit all signals including collisions and noise which limits scalability

  7. Bridges • Connect two segments, but work at the frame level • Use promiscuous mode and forward all frames • Don’t forward erroneous frames (e.g., collisions and noise)

  8. Frame filtering • Only forward a frame if necessary • destination is on the other segment • broadcast address is used • Bridge learns which segment a computer is on when that computer sends a frame • When a frame arrives the bridge • extracts source address and updates knowledge • inspects destination address for forwarding

  9. Example of learning in a bridge

  10. Planning a bridged network • Propagation principle - a bridge will only forward frames as far as is necessary • Bridges allow communication on separate segments to occur at the same time • Plan the network so that computers that communicate frequently are on the same segment • May be possible to improve an existing LAN’s performance by adding a bridge

  11. Bridging between buildings

  12. Bridging longer distances

  13. A cycle of bridges • Compute a distributed spanning tree

  14. Switching • Switch - a single electronic device that transfers frames between computers • Whereas a hub simulates a shared medium, a switch simulates a bridged LAN with one computer per segment • Advantage is greater data rate due to parallelism • Some organisations combine hubs and switches to reduce cost

  15. The concept of a switched LAN

  16. Summary • Fiber optic extensions • Repeaters and multiple repeaters • Bridges • frame filtering • bridging distances • Switches

  17. Summary of LANs • Locality of reference • LAN topologies • MAC protocols, especially CSMA/CD • Hardware addressing and frame types • LAN wiring • Extending LANs

  18. Exam question • http://www.cs.nott.ac.uk/~mvr/2-CCN.doc

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